This invention relates to refractory articles of zirconia partially stabilized with magnesia and particularly to articles which can be made from zirconia of relatively high silica content. The zirconia articles have high strength and toughness at both normal and elevated temperatures, and display good thermal shock resistance over the temperature ranges that prevail at many of the applications to which ceramic refractory articles are normally put.
Because of the sharp volume change undergone by zirconia during its monoclinic/tetragonal transformations, unstabilized zirconia is not generally useful for articles exposed to temperature fluctuations through the range at which the transformation occurs. Consequently, certain stabilizers have been added to and reacted with zirconia to stabilize it, at least partially, in its cubic crystalline form throughout these temperature ranges. Zirconia is "wholly" or "partially" stabilized depending on whether the amount of stabilizer is sufficient to form the cubic phase solid solution with all or part of the zirconia. Partial stabilization is generally sufficient to overcome the destructive thermally-induced volume changes in zirconia. Magnesia, one of the best stabilizers, is effective to partially stabilize zirconia to the desired degree of thermal shock resistance at levels as low as about 2.5-4.0 percent by weight.
One of the important uses for partially stabilized zirconia is as an extrusion die for the high speed, high-temperature extrusion of both ferrous and non-ferrous metals. In this application, extrusion temperatures and pressures can reach 1250.degree. C. and 1500 MPa, respectively. The high stress to which the die material is subjected is evident, and accordingly the material must be physically strong to avoid cracking from thermal and mechanical stresses.
One particularly strong, thermally stable magnesia partially-stabilized zirconia (MgO-PSZ) is shown in U.S. Pat. No. 4,279,655 (Garvie et al; July 21, 1981). The zirconia there disclosed is stabilized with about 2.8-4.0 weight percent magnesia, and through a specific schedule for firing, cooling, and thermal-aging, develops a microstructure characterized by a matrix of grains of cubic-stabilized zirconia wherein each grain contains discrete precipitates of monoclinic zirconia and tetragonal zirconia. This microstructure imparts strength, toughness, and thermal shock resistance, over a wide temperature range.
The reference further teaches, however, that the zirconium dioxide powder from which the stabilized zirconia is prepared must be highly pure, containing no more than about 0.03 weight percent silica, in order for the method of that invention to impart the desired properties to the final zirconia body. This requirement can present practical disadvantages in that the silica level in most commercially available zirconia dioxide powders exceeds that stated maximum. For example, ZIRCOA A (monoclinic) zirconia powder, now available from Corning Glass Works, Corning, N.Y., and made according to U.S. Pat. No. 2,578,748, generally contains from about 0.05-0.5 weight percent silica. Calcia as well as other oxides can also be present in minor amounts as shown in Table 1. The more readily available zirconia powders generally do not meet the purity requirement of U.S. Pat. No. 4,279,655.
TABLE 1 ______________________________________ Chemical Analysis of Typical ZIRCOA-A Zirconia Powder Wt. % ______________________________________ ZrO.sub.2 * 99.60 SiO.sub.2 0.10 CaO 0.17 MgO 0.01 TiO.sub.2 0.04 Al.sub.2 O.sub.3 0.02 Fe.sub.2 O.sub.3 0.04 ______________________________________ *with HfO.sub.2 as natural impurity
Published U.K. Patent Application No. 2,149,773A discloses MgO-PSZ which does contain 0.8-1.5 percent silica. The fired zirconia articles of this publication, however, are not prepared with the aforementioned specific cooling and thermal-aging sequence discussed above, and are not disclosed to attain the beneficial microstructure as a result. Published PCT Application No. WO83/04247 discloses that zirconia powder not meeting the purity requirements of U.S. Pat. No. 4,279,655 can nonetheless be processed according to the method of that patent to attain the desired partially-stabilized microstructure provided that a special additive is introduced to the composition. Specifically, it is disclosed that up to 1.0 weight percent silica is acceptable in the final products when 0.05-1.0 weight percent of a special "additive" (a metal oxide, preferably strontia, baria, or a rare earth oxide, which forms an insoluble zirconate that does not combine with magnesia) is added to the batch before firing. The presence of the additive, however, generally requires prolonged aging time, and the extra cost of that aging, and the possible reduction in high-temperature strength, are disadvantages of this method and composition.
It is therefore an object of the present invention to provide a zirconia ceramic, partially stabilized with magnesia, which has the strength, toughness, and thermal shock resistance imparted by a precipitate-containing microstructure and which can be prepared, if desired, from the readily available batch ZrO.sub.2 powders without the need to introduce special additives or to further purify the batch by removal of silica.